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United States Patent |
5,081,163
|
Pekala
|
January 14, 1992
|
Melamine-formaldehyde aerogels
Abstract
Organic aerogels that are transparent and essentially colorless are
prepa from the aqueous, sol-gel polymerization of melamine with
formaldehyde. The melamine-formaldehyde (MF) aerogels have low densities,
high surface areas, continuous porsity, ultrafine cell/pore sizes, and
optical clarity.
Inventors:
|
Pekala; Richard W. (Pleasant Hill, CA)
|
Assignee:
|
The United States of America as represented by the Department of Energy (Washington, DC)
|
Appl. No.:
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684051 |
Filed:
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April 11, 1991 |
Current U.S. Class: |
521/187; 521/61; 521/64; 528/254 |
Intern'l Class: |
C08G 012/00 |
Field of Search: |
521/61,64,187
528/254
|
References Cited
U.S. Patent Documents
4007142 | Feb., 1977 | Clarke et al. | 521/187.
|
4307201 | Dec., 1981 | Won | 521/187.
|
Primary Examiner: Foelak; Morton
Attorney, Agent or Firm: Carnahan; L. E., Gaither; Roger S., Moser; William R.
Goverment Interests
The United States Government has rights in this invention pursuant to
Contract No. W-7405-ENG-48 between the United States Department of Energy
and the University of California for the operation of the Lawrence
Livermore National Laboratory.
Claims
I claim:
1. A method for producing a melamine-formaldehyde aerogel comprising the
steps of:
a) mixing formaldehyde and melamine in a predetermined ratio wherein there
is an excess of formaldehyde in water in the presence of a base catalyst;
b) heating the mixture to a predetermined temperature for a sufficiently
long period of time to form a clear solution;
c) allowing the solution to cool to a predetermined temperature and then
adding a sufficient amount of an acid to make the solution acidic;
d) allowing the acidic melamine-formaldehyde solution to cure for a
sufficient time at a predetermined temperature to form a gel;
e) placing the gel in a basic solution to neutralize the acid within the
pores of the gel;
f) replacing the aqueous solution within the pores of the gel with a
suitable organic solvent; and
g) critical point drying the gel.
2. A method according to claim 1 comprising the additional step of (h)
characterizing the gel.
3. A method according to claim 1:
a) wherein the formaldehyde and melamine are mixed in a ratio from about
3:1 to about 6:1 moles in the presence of from about 10 to about 100
millimoles of sodium hydroxide;
b) wherein the mixture is heated in step (b) to a temperature of from about
65.degree. C. to about 75.degree. C. for a period of from about five to
about twenty minutes;
c) wherein the solution in step (c) is cooled to from about 40.degree. C.
to about 50.degree. C. and concentrated hydrochloric acid is added to
change the pH to from about to about 2 when measured at about 23.degree.
C.;
d) wherein before step (d), if desired, because of solids in the solution,
the cooled acidic solution from step (c) is reheated in a sealed container
to a temperature of from about 40.degree. C. to about 95.degree. C.;
e) wherein the basic solution of step (e) is ammonium hydroxide;
f) wherein the organic solvent of step (f) is acetone; and
g) wherein in step (g) carbon dioxide is the supercritical fluid.
4. A method according to claim 3 wherein step (f) comprises a series of
solvent exchanges with acetone in the following ratios: a first exchange
with a 50:50 mixture of acetone:water; a second exchange with a 75:25
mixture of acetone:water; and a third exchange with 100% acetone.
5. A method according to claim 3 wherein the pH adjustment of step (c) is
to about 1.5 to about 1.8.
6. A method according to claim 4 wherein 3.7 formaldehyde and 1 mole of
melamine is mixed in step (a).
7. A method according to claim 6 wherein the formaldehyde is methanol
stabilized and at a concentration of 37.6% and wherein the melamine is at
a concentration of greater than 99%; and
wherein the hydrochloric acid used in step (c) is at a concentration of
36.5%.
8. A method according to claim 1 wherein step (e) is omitted.
9. A method according to claim 3 wherein instead of steps (a)-(c), a
melamine-formaldehyde polymer is mixed with water and a sufficient amount
of an acid to effect a pH of from about 1 to about 2.5 and wherein step
(e) is omitted.
10. A method according to claim 9 wherein the pH is from about 1.5 to about
1.8.
11. A method according to claim 3 wherein the curing time in step (d) is
from about 24 hours to about 4 weeks, and the curing temperature is from
about room temperature to about 95.degree. C.
12. A method according to claim 11 wherein the curing time is for about two
days at about 50.degree. C. followed by about five days at 95.degree. C.
13. A method according to claim 9 wherein the curing time is from about 24
hours to about four weeks, and the curing temperature is from was about
room temperature to about 95.degree. C.
14. A method according to claim 13 wherein the curing time is for about two
days at about 50.degree. C. followed by about five days at about
95.degree. C.
Description
BACKGROUND OF THE INVENTION
The invention described herein pertains generally to organic gels and more
specifically to melamine-formaldehyde (MF) aerogels and methods for their
preparation.
Aerogels are a unique class of ultrafine cell size, low density, open-cell
foams. Aerogels have continuous porosity and a microstructure composed of
interconnected colloidal-like particles or polymeric chains with
characteristic diameters of 100 A (angstrom). The microstructure of
aerogels is responsible for their unusual acoustic, mechanical, optical
and thermal properties. [Fricke, Sci. Am., 285(5): 92 (1988); Fricke, in
Sol-Gel Science and Technology (Aegerter et al., eds.) (World Scientific
Publishing, N.J.) 482 (1989).] The microstructure imparts high surface
areas to aerogels, for example, from about 350 m.sup.2 /g to about 1000
m.sup.2 /g. Their ultrafine cell/pore size minimizes light scattering in
the visible spectrum, and thus, aerogels can be prepared as transparent,
porous solids. Further, the high porosity of aerogels makes them excellent
insulators with their thermal conductivity being approximately 100 times
lower than that of the fully dense matrix. Still further, the aerogel
skeleton provides for the low sound velocities observed in aerogels.
[Aerogels (Fricke ed.) (Springer-Verlag N.Y. 1988).]
As a result of their high porosity, aerogels exhibit elastic moduli many
orders of magnitude smaller than their full density analogs. A simple
scaling law relates the aerogel modulus to its density (.rho.), that is,
E=c.rho..sup.n. The scaling constant, n, and prefactor, c, are sensitive
to variations in the aerogel microstructure.
Traditional aerogels are inorganic (for example, silica, alumina or
zirconia aerogels), made via the hydrolysis and condensation of metal
alkoxides, for example, tetramethoxy silane [Teichner et al., Adv. Coll.
Interf. Sci., 5: 245 (1976); Brinker et al., J. Non-Cryst. Solids, 48: 47
(1982); J. Non-Cryst. Solids, 63: 45 (1984)].
Recently, organic aerogels from the sol-gel polymerization of resorcinol
(1,3 dihydroxy benzene) with formaldehyde under alkaline conditions have
been developed as disclosed in U.S. Pat. No. 4,873,218, issued Oct. 10,
1989, to Richard W. Pekala. [Pekala et al., J. de Physique. Colloque
Suppl., 50(4): (4-33) (1989); Pekala, J. Mat. Sci., 24: 3221 (1989);
Pekala and Kong, Polym. Prpts., 30(1): 221 (1989); and Pekala and Stone,
Polym. Prpts., 29(1): 204 (1988).]
Although the resorcinol-formaldehyde aerogels (RF aerogels) exhibit minimal
light scattering, they are dark red in color and have a large absorption
coefficient within the visible spectrum. The color centers present in the
RF aerogels result from oxidation products (for example, quinones) formed
during the polymerization. Their presence has limited the use of the RF
aerogels for certain optical applications where the material needs to
transmit light and be essentially colorless, that is, non-absorptive in
the visible spectrum.
The present invention overcomes the optical limitations of RF aerogels by
providing organic aerogels of low density and high surface area, produced
by the sol-gel polymerization of melamine with formaldehyde; such aerogels
are not only transparent, but also essentially colorless having a slightly
bluish tinge.
The MF aerogels are prepared by the aqueous, sol-gel polymerization of
melamine (2,4,6 triamino s-triazine) with formaldehyde followed by
supercritical extraction. Described herein are processes for preparing MF
aerogels which processes are different from those used to prepare RF
aerogels, primarily in that acidic conditions are necessary to promote
condensation of intermediates in the polymerization process leading to gel
formation. Synthetic conditions, for example, reaction time and pH, affect
the density, transparency and microstructure of the resultant MF aerogels.
Representative densities of the MF aerogels are low from about 100 mg/cc
to about 800 mg/cc, preferably from about 100 mg/cc to about 750 mg/cc;
and the surface area is high, for example, about 1000 m.sup.2 /g.
Kistler described organic foams prepared from nitrocellulose, cellulose,
agar and egg albumin using a supercritical drying procedure. [Nature, 127:
741 (1931).]
Examples of commercially available "low-density" materials are plastic
"blown cell" foams, such as, polyurethane cushions and polystyrene coffee
cups. Asymmetric membranes and filters, on the other hand, are
representative of commercially available "microcellular" materials. The
processes used to make such products are generally not suitable for making
aerogels, however, because they are limited by a trade off between density
and cell size. That is, such processes produce relatively low density
products only at the expense of increased cell size, or produce products
having small cell size at the expense of those products having increased
density. Aerogels, on the other hand, have both low density and small cell
size, as well as meeting other requirements of various applications (for
example, composition, homogeneity, size and strength).
Differentiated from the organic aerogels, such as, RF and MF aerogels, are
the relatively macrocellular (having large cell sizes) foamed organic
polymers and organic foam composite materials that are well-known and used
in the insulation, construction and similar industries. Such foams are not
generally suitable for applications where both very low density and
ultrafine cell sizes are needed, such as in many high-energy physics
applications, or as parts for inertial confinement fusion targets. A
requirement for such organic materials is not only very low density, but
generally at least over an order of magnitude smaller cell size than foams
produced using other conventional techniques such as the expansion of
polymer/blowing agent mixtures, phase-separation of polymer solutions and
replication of sacrificial substrates, to name a few. Some of such prior
art methods have produced phenol-formaldehyde and phenol-urea foams, but
again, such foams have a compact cellular structure, but not the
sufficiently small cell sizes necessary for high-energy physics
applications.
Such materials do not exhibit the desired low density, combined with the
ultrafine cell structure characteristic of aerogels, and are thus not
suitable for applications in high energy physics or as parts for inertial
confinement fusion targets. The current production of low density
materials with ultrafine pore sizes (less than or equal to 1000 A) has
largely been limited to aerogel technology, particularly to silica
aerogels.
Silica aerogels are being developed as superinsulating material for double
pane windows. Organic aerogels would be expected to have an even lower
thermal conductivity and, thus, provide less heat loss in insulating
applications.
The presence of silicon, having an atomic number (Z) of 14, in silica
aerogel systems often limits its effectiveness for many applications, such
as in high energy physics or as parts for inertial confinement fusion
targets and the like, where a low number for Z (atomic number) is
preferred. Pure organic foams or aerogels, consisting of mostly carbon
(Z=6), and hydrogen (Z=1) with some oxygen (Z=8), are suitable for such
applications. The organic composition of MF aerogels provides them with a
low average atomic number, making them ideal candidates for high energy
physics applications and as parts for inertial confinement fusion targets.
Other potential applications for the MF aerogels of this invention include,
but are not limited to, uses as catalyst supports, permselective
membranes, thermal insulators, gas filters in chemical processing
chromatographic packings, sensors, lenses, solar collectors and impedance
matching devices. Future applications could include lightweight insulative
clothing, fire-retardant architectural materials, high resolution sonic
detectors, autofocus cameras, dielectric spacer material for electronics
and magnetics, acoustic and thermal absorbers for packaging valuable
temperature-sensitive products.
Accordingly, it is an object of the present invention to provide a low
density organic aerogel which exhibits continuous porosity and ultrafine
cell size and is not only transparent, but also essentially colorless,
that is, non-absorptive in the visible spectrum.
Another object of the invention is to provide a new synthetic route for the
production of organic aerogels. The aqueous, sol-gel polymerization of
melamine with formaldehyde requiring a pH change, followed by
supercritical extraction, lead to the formation of a new type of organic
aerogel. Low densities (from about 0.1 to about 0.8 g/cc), high surface
areas (about 1000 m.sup.2 /g) and optical clarity are only a few of the
characteristics of the MF aerogels of this invention.
Additional objects, advantages and novel features of the invention,
together with additional features contributing thereto and advantages
accruing therefrom will be apparent from the following description and the
accompanying illustration of one or more embodiments of the invention and
the description of the preparation techniques therefor, as described
hereinafter. The objects and advantages of the invention may be realized
and attained by means of the instrumentalities and combinations
particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with the
purpose of the present invention as embodied and broadly described herein,
one aspect of the present invention provides, a synthetic route for the
production of melamine-formaldehyde aerogels which are essentially
colorless, exhibit optical clarity, low densities, continuous porosity and
ultrafine cell size and a Z =8 or less, as well as MF aerogels with such
characteristics. The method broadly comprises the sol-gel polymerization
of melamine with formaldehyde to form an organic aerogel which is both
colorless and transparent. The polycondensation of formaldehyde with
melamine is carried out in water in the presence of a base catalyst.
Sodium hydroxide is a suitable base catalyst, although other catalysts may
be employed. The slurry is heated to form a clear solution and upon
cooling acidified, wherein the acid used is preferably hydrochloric or
trifluoroacetic acid among others. To form transparent gels when
formaldehyde and melamine monomers are used as starting materials, the pH
of the solution is maintained in a range of from about 1 to about 2,
preferably between about 1.5 and about 1.8. Outside of that pH range,
translucent or opaque gels are formed. When a low molecular weight
melamine-formaldehyde polymer is used as the starting material, the pH
range can be broadened to from about 1 to about 2.5.
A schematic diagram of the melamine-formaldehyde reaction is shown in FIG.
1. In preparation for supercritical drying, the gels can be placed in an
appropriate basic solution to neutralize the acidic solution within the
pores of the gel; however, such a neutralization step is not necessary
when a low molecular weight melamine-formaldehyde polymer is used as the
starting material. The gel is then exchanged into a suitable organic
solvent and supercritically dried with carbon-dioxide.
The melamine-formaldehyde aerogels can be prepared from melamine and
formaldehyde monomers or from melamine-formaldehyde low molecular weight
polymers, preferably CYMEL 385 [American Cyanamid, Wayne, N.J. (USA)] and
Resimene.RTM. 714 [Monsanto Chem. Co., St. Louis, MO (USA)].
The MF aerogels of this invention are stable, non-absorptive in the visible
spectrum, transparent and exhibit low densities (from about 0.1 to about
0.8 g/cc), high surface areas (about 1000 m.sup.2 /g), and have ultrafine
cell pore sizes (less than or equal to 1000 A, more preferably less than
or equal to 500 A). Their organic composition provides a low average
atomic number that provides certain advantages over conventional inorganic
aerogels, such as, silica or alumina aerogels.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic diagram of the reaction of melamine with formaldehyde
to form a cross-linked polymer network.
FIG. 2 shows a melamine/formaldehyde (MF) aerogel after supercritical
drying. The aerogel is 17 mm thick, colorless and transparent with a
density of 0.3 g/cc.
FIG. 3 is a scanning electron micrograph showing the fracture surface of a
MF aerogel. The micrograph reveals that the aerogel is composed of
interconnected particles or fibers (cross-linked polymeric chains) with
diameters less than 500 angstroms (A).
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to melamine-formaldehyde (MF) aerogels,
which are essentially colorless and transparent; have low densities,
preferably in the range of about 0.1 to about 0.8 g/cc, more preferably
from about 0.1 to about 0.75 g/cc; have high surface areas, from about 900
m.sup.2 /g to about 1100 m.sup.2 /g, generally about 1000 m.sup.2 /g; and
have ultrafine cell/pore size, preferably less than about 1000 A, and more
preferably less than or equal to 500 A.
Further, the invention is directed to methods for producing such MF
aerogels and MF aerogels produced by such methods.
The aqueous, sol-gel polymerization of melamine with formaldehyde, followed
by supercritical extraction, leads to the formation of a new type of
organic aerogel--the MF aerogel. Synthetic conditions, for example, the
reaction time and pH, affect the density, transparency, and microstructure
of the resultant MF aerogels. Unlike the previous organic aerogels based
upon resorcinol-formaldehyde, the MF aerogels are both transparent and
colorless.
Melamine is a hexafunctional monomer capable of reaction at each of the
amine hydrogens. Under alkaline conditions, formaldehyde adds to the above
positions to form hydroxymethyl (--CH.sub.2 OH) groups. In the second part
of the polymerization, the solution is acidified to promote condensation
of these intermediates, leading to gel formation. The principal
crosslinking reactions include the formation of (1) diamino methylene
(--NHCH.sub.2 NH--) and (2) diamino methylene ether (--NHCH.sub.2
OCH.sub.2 NH--) bridges [Blank, J. Coatings Tech., 51 (656): 61 (1979);
and Updegraff, "Amino Resins," in Encyclopedia of Polymer Science (2d
ed.), Vol. 1: 752-789 (John Wiley & Sons, 1985)]. FIG. 1 outlines the MF
reaction and depicts the formation of a crosslinked polymer network.
Methods to produce such MF aerogels comprise reacting melamine and
formaldehyde in an appropriate ratio, preferably from about 3 to about 6
moles of formaldehyde with about 1 mole of melamine, more preferably about
3.7 moles of formaldehyde to about 1 mole of melamine, to form a
crosslinked network. An excess of formaldehyde is preferably used to
maximize the crosslinking density of the gel.
In addition to forming MF aerogels from melamine and formaldehyde monomers,
low molecular weight polymers that are commercially available can be
employed. Preferred polymers include CYMEL 385 [a partially methylated
melamine--formaldehyde resin with a methoxymethyl-methylol functionality;
commercially available from American Cyanamid Company, Wayne, NJ (USA)]
and Resimene.RTM. 714 [a partially methylated melamine-formaldehyde resin
also with a methoxymethyl-methylol functionality; commercially available
from Monsanto Chemical Co., St. Louis, MO (USA).] Example 2 provides a
representative method of making MF aerogels with such resins.
A method of producing MF aerogels comprises reacting melamine and
formaldehyde monomers in a predetermined ratio, in the presence of a base
catalyst, preferably sodium hydroxide, in an aqueous solution at an
elevated temperature (because of melamine's limited water solubility),
preferably about 65.degree. C. to about 75.degree. C., more preferably
about 70.degree. C., for a sufficient amount of time, preferably about 5
to about 20 minutes, more preferably about 10 to about 15 minutes, to form
a clear solution. The solution is allowed to cool, preferably to about
40.degree. C. to about 50.degree. C., more preferably to about 45.degree.
C., and then sufficient acid, preferably hydrochloric acid (HCl) or
trifluoroacetic acid (TFAA), preferably concentrated hydrochloric acid
(HCl), more preferably at a concentration of about 36.5%, is added to
cause the solution to be in an acidic pH range as measured at room
temperature, preferably from about 1 to about 2, more preferably from
about 1.5 to about 1.8 when melamine and formaldehyde monomers are
employed, and preferably from about 1 to about 2.5 when low molecular
weight melamine-formaldehyde polymers are used. If the solution is heated
too long or the pH is not properly adjusted, a white precipitate can be
formed, and a gel will not be formed.
To form transparent gels, the pH of the solution needs to be in the range
of from about 1 to about 2, or about 1 to 2.5 (as indicated above
depending upon the starting material), when measured at 23.degree. C.
Outside of those pH ranges, translucent or opaque gels are formed. The pH
of the MF solution appears to be the most critical parameter in
controlling the optical clarity of the dried aerogel. Gels prepared from
monomers at a pH of about 1.7 resulted in transparent aerogels, whereas
gels prepared at a pH of about 0.7 led to opaque aerogels. Infrared (IR)
spectra of those two aerogels indicated that the absorption rates and
intensity ratios thereof were identical. Solid state nuclear magnetic
resonance (NMR) of these aerogels also showed identical chemical shifts
and relaxation parameters. Thus, based upon the IR and NMR data, it
appears that the solution pH does not affect the type or degree of
crosslinking in the aerogels.
Depending upon the percentage of solids in the pH adjusted solution
(acidic), it may be necessary to reheat the solution to a temperature
preferably from about 40.degree. C. to about 95.degree. C. in a sealed
container to form a gel.
The pH adjusted, melamine-formaldehyde solution is then poured into an
appropriate container, preferably glass vials, sealed and cured under
various conditions. Solutions that contained greater than or equal to 20%
reactants gelled in less than 48 hours at room temperature; whereas
solutions containing about 7% reactants gelled in approximately 4 weeks at
a cure temperature of from about 85.degree. C. to about 95.degree. C. It
was found that at high reactant concentrations, that is, from about 15% to
about 40% reactants, a preferred curing pattern was for about 50.degree.
C. for about two days followed by a curing time of about five days at an
elevated temperature that is less that the boiling point of water,
preferably about 95.degree. C.
As the reaction progresses, all formulations acquire a light blue haze.
That haze is associated with Rayleigh scattering from MF "clusters"
generated in solution. The clusters contain surface functional groups, for
example, --CH.sub.2 OH, that eventually crosslink to form a gel. The
aggregation and crosslinking processes show a strong pH dependence.
In preparation for supercritical drying, the crosslinked gels that are
prepared from melamine and formaldehyde monomers are removed from their
containers and placed in a basic solution, preferably ammonium hydroxide,
to neutralize the HCl within their pores. Such a neutralization step was
found not to be necessary when low molecular weight melamine-formaldehyde
polymers are used as the starting material, and may not be necessary when
melamine and formaldehyde monomers are used as the starting materials. The
gel is then solvent exchanged to replace the water retained in the gel
pores with a suitable organic solvent. Such solvents include, but are not
limited to, methanol, acetone, isopropanol and amyl acetate, wherein
acetone is the preferred solvent. For example, the gels are exposed to a
50:50 mixture of acetone:water, followed by a 75:25 mixture and finally
100% acetone. Multiple exchanges with fresh acetone are used to remove
residual water from the gels.
After the solvent exchange, the gel is dried by supercritical drying, using
carbon dioxide, and characterized, for example, in terms of density,
microcellular structure or pore size and spectral characteristics. Details
concerning the supercritical extraction procedure can be found in Pekala
and Kong, J. de Physique, Colloque Suppl., 50(4): C4-33 (1989) and Pekala,
J. Mat. Sci., 24: 3221 (1989). Briefly and representatively, wherein
carbon dioxide is the supercritical fluid, the solvent-filled, preferably
acetone-filled gel, is placed in a temperature-controlled, pressure vessel
(for example, Poloron.RTM., Watford, England), which vessel is then filled
with liquified carbon dioxide. The carbon dioxide is completely
substituted for the acetone in the pores of the gel through multiple
exchanges. At that point, the vessel is heated above the critical
temperature (T.sub.c =31.degree. C.) and brought to a pressure above the
critical pressure (P.sub.c =1100 psi) of carbon dioxide. The pressure is
then slowly bled from the vessel while the temperature is maintained above
the critical point. At atmospheric pressure, the MF aerogel is removed
from the vessel.
The MF aerogels so formed are transparent, indicative of their ultrafine
cell/pore size (less than 1000 A, preferably less than 500 A). All samples
were stored in dessicators to inhibit moisture absorption.
MF aerogels have been synthesized with densities from about 0.1 to about
0.75 g/cc. FIG. 2 shows an MF aerogel after supercritical drying. The
aerogel is both colorless and transparent. The latter property indicates
that the cell/pore size and characteristic particle size (referred to as
"cluster" size in solution) are less than 1/20th the wavelight of visible
light. The optical clarity of MF aerogels is equivalent to that of many
silica aerogels.
The fracture surface of an MF aerogel is shown in FIG. 3. Scanning electron
microscopy (SEM) reveals that the aerogel is composed of interconnected
particles with diameters less than 500 A. At that magnification, it is
difficult to discern whether the particles are composed of even smaller
subunits. Measurements of surface area by the Brunauer, Emmett and Teller
nitrogen absorption method (BET) gave a surface area of 970 m.sup.2 /g.
Aerogel moduli were measured in uniaxial compression with an Instron
machine [Model #1125; Instron Corp., 100 Royall Street, Canton, MA 02021
(USA)]. Tests were performed at a strain rate of 0.1%/second under ambient
conditions. Specimens were machined as 1.times.1.times.1 cm.sup.3 cubes
with a modified end mill. Great care was taken to ensure that specimens
were machined with flat, smooth surfaces and plane-parallel opposing
faces. Densities were measured just prior to testing, and the compressive
modulus was derived from the linear region of the stress-strain curve.
In order to investigate the structure-property relationships of the MF
aerogels, compressive moduli were examined as a function of density and
compared to silica aerogels. As expected, the modulus increases with
aerogel density. The linear log-log plot in each case demonstrates a
power-low density dependence that has been observed in many other low
density materials. That relationship is expressed as E=c.rho..sup.n, where
.rho. is the bulk density, c is a prefactor (constant), and n is a
non-integer scaling exponent that actually ranges from 2-4 [Gibson and
Ashby, Proc. Roval Soc. Land., 382 (A): 43 (1982)].
For silica aerogels, the scaling exponent shows a strong dependence upon
catalyst conditions [Woignier et al., J. Mat. Res., 4 (3): 688 (1989);
LeMay et al., Pac. Polym. Prpts., 1: 295 (1989)]. Transparent MF aerogels
have a scaling exponent of 3.3 plus or minus 0.3 and moduli that
approximate those acid-catalyzed silica aerogels. That data suggest
similar microstructures for the two aerogels, even though MF aerogels are
organic and silica aerogels are inorganic. MF aerogels, thus, were found
to be similar to silica aerogels in terms of their microstructure, surface
area and mechanical properties.
MF aerogels have moduli that are similar to RF aerogels synthesized under
high catalyst conditions [Pekala et al., in Mechanical Properties of
Porous and Cellular Materials (Gibson et al. eds.), MRS Symp. Proc., 207
(in press)]. The scaling exponent of MF aerogels differs from RF aerogels,
which is not surprising because MF aerogels are produced under highly
acidic conditions, whereas RF aerogels are base catalyzed. The higher
exponent implies a different microstructure as would be expected from the
cluster-cluster growth pathway of the MF polymerization.
The following representative examples are illustrative of the principles of
the present invention and describe preferred embodiments thereof. They are
not to be construed as limiting the invention in any manner or to any
precise form.
EXAMPLE 1
The polycondensation of 3.7 moles of formaldehyde [37.6%; methanol
stabilized; J. T. Baker, Phillipsburg, NJ 08865 (USA)] with 1 mole of
melamine [99+%; Aldrich Chemical Co., 1001 W. St. Paul Ave., Milwaukee, WI
53233 (USA)] was carried out in deionized and distilled water using 10-100
millimoles of sodium hydroxide as a base catalyst. The slurry formed was
heated for 10-15 minutes at 70.degree. C. to form a clear solution. This
solution was allowed to cool to 45.degree. C., at which time, concentrated
hydrochloric acid (HCl) (36.5%) was added. If the solution was heated too
long at 70.degree. C. or the pH was not properly adjusted a white
precipitate was formed and a gel could not be obtained. To form
transparent gels, the pH of the melamine-formaldehyde (MF) solution was
required to be from about 1 to about 2 more preferably from about 1.5 to
about 1.8 when measured at 23.degree. C. Outside of that range,
translucent or opaque gels were formed.
The pH adjusted, melamine-formaldehyde solution was poured into 23.times.85
mm glass vials, sealed, and cured under various conditions. Solutions
containing greater than or equal to 20% reactants gelled in less than 48
hours at room temperature, whereas solutions containing 7% reactants
gelled in approximately 4 weeks at a cure temperature from about
85.degree. C. to about 95.degree. C. As the reaction progressed, all
formulations acquired a light blue haze.
In preparation for supercritical drying, the crosslinked gels were removed
from their glass vials and placed in an ammonium hydroxide solution to
neutralize the HCl within the pores. The gels were then exposed to 50:50
mixture of acetone:water, followed by a 75:25 mixture, and finally 100%
acetone. Multiple exchanges with fresh acetone were used to remove
residual water from the gels.
The acetone-filled, MF gels were dried in a jacketed pressure vessel
(Polaron Equipment Ltd., Watford, England) using carbon dioxide as the
supercritical fluid (T.sub.c =31.degree. C.; P.sub.c =1100 psi). The
carbon dioxide was completely substituted for the acetone in the pores of
the gel through multiple exchanges. The vessel was heated above the
critical pressure and temperature of carbon dioxide. The pressure was
slowly bled from the vessel while the temperature was maintained above the
critical point. At atmospheric pressure, the MF aerogels were removed from
the vessel. [Further details concerning the supercritical extraction
procedure can be found in Pekala and Kong, J. de Physique, Colloque
Suppl., 50 (4): C4-33 (1989) and in Pekala, J. Mat. Sci., 24: 3221
(1989).]
The MF aerogels removed from the pressure vessel were transparent,
indicative of the ultrafine cell size (less than 500 A) of these porous
solids. All samples were stored in dessicators to inhibit moisture
absorption.
EXAMPLE 2
To make an approximately 200 mg/cc melamine-formaldehyde aerogel with a
commercially available melamine-formaldehyde polymer, the following
formulation is used:
12.5 g of Resimene.RTM. 714 (Monsanto Chem. Co., St. Louis, MO);
0 52.5 g of deionized and distilled water; and
about 2 ml of concentrated hydrochloric acid (HCl) to bring the pH to about
1.75.
The solution is stirred and poured into glass molds. The formulation is
cured for two days at about 50.degree. C. followed by about five days at
about 95.degree. C.
The solvent exchange procedure except without the neutralization step, as
outlined above in Example 1, is followed and the gel is supercritically
dried from carbon dioxide. A transparent aerogel results.
It has thus been shown that melamine formaldehyde aerogels of microcellular
structure are easily produced in low densities ranging from about 0.1 to
about 0.8 g/cc. These melamine formaldehyde aerogels are essentially
colorless, transparent, and have high surface areas and ultrafine
cell/pore sizes.
The foregoing description of a preferred embodiment of the invention has
been presented for purposes of illustration and description only. It is
not intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations are
possible in light of the above teaching. The particular embodiment was
chosen and described in order to best explain the principles of the
invention and its practical application thereby to enable others skilled
in the art to best utilize the invention in various embodiments and with
various modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the claims
appended hereto. All references herein cited are hereby incorporated by
reference.
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